Onions produce disulfides when digested which are toxic to birds, potentially causing much the same problems that they do in dogs. I guess the question for those interested in further digging would be how much and how often they would have to be consumed to cause a problem.
I'm not sure if it is safe but I wouldn't because it will give the eggs and meat (if they are meat birds) an off taste. The same is true for garlic and fish meal.
My hen raced towards the green onion stem before my rooster could! I believe it IS safe, but I will wait until she lays next morning to see if it ruins the taste!